Extruding generally involves forcing a deformable material in a continuous stream through a die to form a profiled extrusion. An example of a conventional extruder is shown in FIG. 1. Extruder 10 generally includes a body 12 having a longitudinal bore 14 therein where the material 30 to be extruded enters the apparatus from a supply 16 of such material. An auger 18 is then rotated to force material 30 along the length of bore 14 and through a die opening 20 located at the distal end of bore 14 to form a profiled extrusion. For example, a continuous sheet may be extruded through a die having a slot-shaped profile corresponding to the desired width and thickness of the sheet, as shown in FIG. 1. The extrusion may then be processed as desired.
Some materials are more easily extruded than other materials. For example, a Newtonian fluid such as a petroleum oil may flow through the extruder die more easily and more uniformly than a Non-newtonian fluid such as a paste, clay, or gel. The present invention relates to extrusion of materials known as Bingham plastic-type materials, which exhibit a yield stress that must be exceeded before the material will deform. Stated differently, at relatively low stress levels the material will behave as a solid, and above the yield stress the material will deform. Bingham plastic-type materials include, but are not limited to materials such as gels, pastes, sol gel ceramic precursors, silicas, and clays.
When Bingham plastic-type materials are extruded through a conventional die having an elongate die opening, the results are not uniformly satisfactory. For example, the Bingham plastic-type material may channel, meaning that it flows from less than the entire width of the die, because there is insufficient localized pressure to force the material to the outer edges of the die opening. Channeling thus results in a nonuniform sheet. Furthermore, the extrusion of Bingham plastic-type materials requires a powerful pumping device to induce flow of the material through the die opening by exceeding the yield stress of the material.
One attempt to solve the problems described above was the use of multiple extruders 10 operating simultaneously and extruding material through a single die 20', as shown in FIG. 2. Although this approach has some utility from the standpoint of sheet uniformity, multiple extruders are expensive to purchase, operate, and maintain, and they occupy proportionately more floor space in the shop. The material extruded from each extruder may still channel, resulting in a non-uniform sheet as discussed above with reference to a single extruder.
It is therefore desirable to provide a reliable, inexpensive method and apparatus for extruding a uniform sheet from a Bingham plastic-type material.